Sane strict configuration for golangci-lint.

Enabled all linters/rules except ones which will likely hurt readability (e.g. because they recommend some premature optimization) or too annoying in real projects without enough value.

This is recommended config for a new projects, but existing projects may require some fair amount of work to start using this config.

There are 2 versions of each config: VERSION/.golangci.yml and VERSION/reference/.golangci.yml. They are the same (first one is generated from second one using scripts/convert).